SCN9A (Sodium Voltage-Gated Channel Alpha Subunit 9): SCN9A encodes a voltage-gated sodium channel (NaV1.7) that is highly expressed in the peripheral nervous system, particularly in pain-sensing neurons (nociceptors). Mutations in SCN9A can lead to either gain-of-function or loss-of-function effects, resulting in conditions like erythromelalgia (extreme pain and redness in extremities) or congenital insensitivity to pain, respectively. This channel is a critical modulator of pain sensation, making it a target for the development of novel analgesics....